With SPI flash moving to driver model, commit fbb09918 "dm: Convert
spi_flash_probe() and 'sf probe' to use driver model" ignored the
SST flash-specific write op (byte program & word program), which
actually broke the SST flash from wroking.

This commit makes SST flash work again under driver model, by adding
SST flash-specific handling in the spi_flash_std_write().

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>